With the increase in the demand for electric power in the present society, the demand for power cables has increased. As a result, power cables that are capable of transmitting high-capacity electric power have been actively developed.
In general, a power cable includes a conductor and an insulation layer for covering the conductor. A high-voltage or ultrahigh-voltage cable may further include an inner semi-conductive layer interposed between the conductor and the insulation layer, an outer semi-conductive layer for covering the insulation layer, and a sheath layer for covering the outer semi-conductive layer. After being used for a predetermined period of time, the power cable is discarded. That is, the power cable is an article of consumption. A material obtained by cross-linking polyolefin polymers, such as polyethylene, ethylene polyethylene rubber (EPR), and ethylene propylene diene monomer (EPDM), which exhibit excellent mechanical and electrical characteristics, is generally used as a conventional insulation material for the power cable.
For cross-linked polyethylene (XLPE), however, it is not possible to reshape an insulation layer that does not satisfy quality control (QC) standards, such as cross-linkability, after the production of a cable. As a result, the produced insulation layer is discarded. In addition, it is difficult to burn up cables after the use of the cables for the lifetime thereof. That is, the cables are not environmentally friendly. Furthermore, in order to manufacture a cross-linked polyethylene (XLPE) cable, a cross-linking process at a high temperature and a high pressure (for example, about 300° C. and 7 atmospheres) is required after the formation of the cable. To this end, it is essentially required to introduce a curing pipe more than several tens of meters long, with the result that the costs related to a manufacturing factory site and necessary electric power are considerably increased. Furthermore, it is essentially required to add an expensive chemical material (e.g. a cross-linking agent) for cross-linking, with the result that manufacturing costs and resources consumption are increased.
Various attempts have been made in order to solve the above-mentioned problems. For example, Korean Patent Application Publication No. 10-2014-0134836 discloses a composition configured such that propylene copolymer particles are dispersed in a polypropylene matrix, wherein the composition is used as an insulation material. An environmentally friendly polypropylene resin that exhibits high heat resistance without being cross-linked since the melting point of the polymer is 160° C. or higher is used as a matrix, and a propylene copolymer having a predetermined average particle size is dispersed in the polypropylene matrix in order to improve the low flexibility and bendability of the polypropylene resin, which are the drawbacks of the polypropylene resin. The resultant composition is used as an insulation material. The product manufacturing process is completed immediately after the formation of the cables without an additional cross-linking process, with the result that manufacturing costs are considerably reduced. In addition, in the event that a produced cable does not satisfy prescribed standards, the insulation layer may be separated and reshaped, with the result that the produced insulation layer may be reused.
However, the above-mentioned publication does not suggest the possibility of a heterophasic polymer composition for cable insulation layers that exhibits higher electrical characteristics than conventional heterophasic polymer compositions and, in addition, exhibits high dielectric strength at a high temperature or when mechanical force is applied thereto.
In addition, with the increase in the amount of electric power that is used, power cables capable of transmitting high-voltage electricity have been increasingly used, and there is an urgent necessity to improve the properties of polypropylene in order to positively use polypropylene as an environmentally friendly insulation layer that retains its electrical characteristics even at a high temperature. In conclusion, there is a high necessity for a heterophasic polymer composition for cable insulation layers that exhibits further improved AC dielectric breakdown strength, long-term heat resistance, electrical insulation, and mechanical strength while exhibiting flexibility and bendability equal to or higher than those of cross-linked polyethylene (XLPE) such that the cross-linked polyethylene (XLPE) can be replaced with the heterophasic polymer composition.